Optical writing device, image forming apparatus, and temperature calculation method

ABSTRACT

An optical writing device having; a plurality of light-emitting points; a photodiode configured to output a signal which represents a quantity of incident light from a predetermined light-emitting point selected from the plurality of light-emitting points; and a calculation section for calculating a temperature of the photodiode based on a magnitude of a photodiode dark current included in the signal output from the photodiode while the predetermined light-emitting point is OFF.

This application is based on Japanese Patent Application No. 2013-213715filed on Oct. 11, 2013, the content of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical writing device including aphotodetector for detecting the quantity of light emitted from alight-emitting point, an image forming apparatus, and a temperaturecalculation method.

2. Description of Related Art

In an image forming apparatus, an optical writing device is used forformation of images. One of known examples of the invention relating toconventional optical writing devices is a light-emitting devicedisclosed in Japanese Patent Laid-Open Publication No. 2006-201751. Thelight-emitting device includes a light-emitting element for lightquantity detection, which is not provided for an intended use and whichis exclusively used for detection of light, a light quantity detectorfor detecting the quantity of light from the light-emitting element forlight quantity detection, and a corrector for correcting the drivingconditions of a light-emitting element based on the quantity of lightfrom the light-emitting element for light quantity detection such thatthe variation of the quantity of light from the light-emitting elementis suppressed.

In the light-emitting device disclosed in Japanese Patent Laid-OpenPublication No. 2006-201751, as the temperature increases, the quantityof light from the light-emitting element for light quantity detectionalso increases. As the temperature decreases, the quantity of light fromthe light-emitting element for light quantity detection also decreases.That is, the light-emitting element for light quantity detection has atemperature dependence. In view of such, the corrector determines anincrease or decrease of the temperature of the light-emitting elementfor light quantity detection based on the quantity of light from thelight-emitting element for light quantity detection which is detected bythe light quantity detector, and corrects the driving voltage of thelight-emitting element for light quantity detection. With thisconfiguration, in the light-emitting device disclosed in Japanese PatentLaid-Open Publication No. 2006-201751, the variation of the lightquantity which is attributed to the variation of the temperature of thelight-emitting element can be corrected.

A light quantity detector such as a photodetector has such acharacteristic that, even when the intensity of incident light is equal,the level of the output varies depending on the temperature of the lightquantity detector. That is, the light quantity detector has atemperature dependence. On the other hand, in the light-emitting devicedisclosed in Japanese Patent Laid-Open Publication No. 2006-201751, thetemperature dependence of the light-emitting element is considered, butthe temperature dependence of the light quantity detector thatcorresponds to the light-emitting element is not considered. Therefore,in the light-emitting device disclosed in Japanese Patent Laid-OpenPublication No. 2006-201751, it is difficult for the light quantitydetector to accurately detect the quantity of light from thelight-emitting element for light quantity detection. Without accuratedetection of the quantity of light from the light-emitting element forlight quantity detection, the quantity of light used for writing varies,and an image formed in the image forming apparatus deteriorates.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical writingdevice in which the temperature of a photodetector can be determined, animage forming apparatus, and a temperature calculation method.

An optical writing device according to one embodiment of the presentinvention includes: a plurality of light-emitting points; a photodiodeconfigured to output a signal which represents a quantity of incidentlight from a predetermined light-emitting point selected from theplurality of light-emitting points; and a calculation section forcalculating a temperature of the photodiode based on a magnitude of aphotodiode dark current included in the signal output from thephotodiode while the predetermined light-emitting point is OFF.

An image forming apparatus according to another embodiment of thepresent invention includes the above-described optical writing device.

A temperature calculation method according to still another embodimentof the present invention is a temperature calculation method fordetermining, in an optical writing device including a plurality oflight-emitting points, a temperature of a photodiode configured tooutput a signal which represents a quantity of incident light from apredetermined light-emitting point selected from the plurality oflight-emitting points, the method including the steps of; acquiring asignal from the photodiode while the predetermined light-emitting pointis OFF; and determining the temperature of the photodiode based on amagnitude of a photodiode dark current included in the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an image forming apparatus whichincludes an optical writing device of one embodiment.

FIG. 2A is a vertical cross-sectional view of an OLED-PH shown in FIG.1.

FIG. 2B is a graph showing the quantity of light emitted from alight-emitting point shown in FIG. 2A with respect to the drive current.

FIG. 3 is a first block diagram showing details of the configuration ofthe OLED-PH and surrounding components.

FIG. 4A is a second block diagram showing details of the configurationof the OLED-PH and surrounding components.

FIG. 4B is a graph showing the output value (output current) of aphotodetector shown in FIG. 4A with respect to the quantity of incidentlight.

FIG. 5A is a circuit diagram showing details of the configuration of aphotodetector, an integrating circuit, and a S/H circuit shown in FIG. 3and FIG. 4A.

FIG. 5B is a circuit diagram showing details of the configuration of asubtracting circuit shown in FIG. 3 and FIG. 4A.

FIG. 5C is a vertical cross-sectional view showing details of theconfiguration of the photodetector shown in FIG. 3 and FIG. 4A.

FIG. 6 is a flowchart of an operation carried out by a control circuitof the OLED-PH.

FIG. 7 is a timing chart of a temperature correction operation.

FIG. 8 is a subroutine of a gain setting operation of step S02 of FIG.6.

FIG. 9 is a timing chart of the gain setting operation.

FIG. 10 is a timing chart of a light quantity correction operation.

FIG. 11 is a timing chart showing a procedure of light quantitydetection on a group-by-group basis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Introduction

Hereinafter, an image forming apparatus capable of adopting any one ofthe optical writing devices according to embodiments of the presentinvention, and a temperature detection method, are described in detailwith reference to the drawings.

First, the x-axis, the y-axis and the z-axis in the drawings aredescribed. In the following description of the embodiments, forconvenience sake, the x-axis, the y-axis and the z-axis are theright-left direction (horizontal direction), the front-back direction(depth direction) and the up-down direction (vertical direction). In thedrawings, some elements are denoted by reference numbers with suffixesa, b, c and d attached thereto. The suffixes a, b, c and d mean yellow(Y), magenta (M), cyan (C) and black (Bk), respectively. For example, animage forming unit 29 a means an image forming unit 29 for yellow. Areference number with no suffix means an element for each of the colorsY, M, C and Bk. For example, an image forming unit 29 means each of theimage forming units respectively for the colors Y, M, C and Bk.

Structure and Operation of the Image Forming Apparatus

FIG. 1 is a schematic diagram showing an image forming apparatus 1 whichincludes an optical writing device of one embodiment. FIG. 2A is avertical cross-sectional view of an OLED-PH 17 shown in FIG. 1. FIG. 2Bis a graph showing the quantity of light emitted from a light-emittingpoint shown in FIG. 2A with respect to the drive current. In FIG. 1, theimage forming apparatus 1, which is, for example, an MFP (multifunctionperipheral), forms toner images in the respective colors by usingphotoreceptor drums 31 for the respective colors, combines the tonerimages to form a composite toner image and prints the composite tonerimage on a sheet S. For this purpose, the image forming apparatus 1generally comprises a sheet feed unit 3, a pair of timing rollers 5, aprocess unit 7, a fixing device 9, a pair of ejection rollers 11 and aprinted-sheet tray 13.

The sheet feed unit 3 comprises a feed tray 15 and feed roller 16. Onthe feed tray 15, a plurality of sheets S to be printed are stacked. Thefeed roller 16 picks up the topmost one from the stack of sheets S andfeeds the sheet S into a sheet path R. The sheet S is fed toward thepair of timing rollers 5 located immediately downstream.

The pair of timing rollers 5 comprises a pair of rollers in contact witheach other on the sheet path R. The pair of timing rollers 5 are rotatedand stopped under control of a control circuit 37. The pair of timingrollers 5 stays in a stopped state except for the time of feeding thesheet S. Accordingly, the sheet S fed to the pair of timing rollers 5runs head-on into a contact portion of the timing rollers 5, whereby thesheet S stops. Thereafter, the pair of timing rollers 5 starts rotatingat predetermined timing to feed the sheet S toward a secondary transferarea, which will be described later.

The process unit 7 comprises OLED-PHs 17 for the respective colors,transfer devices 19 for the respective colors, an intermediate transferbelt 21, a drive roller 23, a driven roller 25, a secondary transferroller 27, and image forming units 29 for the respective colors. Each ofthe image forming units 29 generally comprises a photoreceptor drum 31,and a set of a charger 33 and a developing device 35 arranged along theperipheral surface of the photoreceptor drum 31.

Each of the photoreceptor drums 31 for the respective colors extends inthe y-axis direction. These photoreceptor drums 31 are arranged in thex-axis direction. Each of the photoreceptor drums 31 is driven by amotor (not shown) to rotate clockwise (shown by arrow CW) in a zx-planeabout an axis in parallel to the y-axis direction.

Each of the chargers 33 extends in the y-axis direction and charges theperipheral surface of the corresponding photoreceptor drum 31. Thechargers 33 are typically corotron chargers, scorotron chargers orcharging rollers.

Each of the OLED-PHs 17 is an optical writing device located near theperipheral surface of the corresponding photoreceptor drum 31 andimmediately downstream of the corresponding charger 33 in the rotatingdirection CW of the photoreceptor drum 31. Each of the OLED-PHs 17, asshown by FIG. 2A, comprises at least an OLED substrate 52 and a lensarray 53 held in a holder 51.

The holder 51 extends in parallel to the corresponding photoreceptordrum 31 and is opposed to an exposure position where the correspondingphotoreceptor drum 31 is exposed to a light beam B.

The OLED substrate 52, as shown in FIG. 2A, supports light-emittingpoints A1 to A4, B1 to B4, C1 to C4 . . . , of which number is equal tothe number of dots in one line in the y-axis direction. (For example,ten thousands and several thousands of light-emitting points areprovided).

Each of the light-emitting points A1 to A4, B1 to B4, C1 to C4 . . . istypically an OLED, and the light-emitting points A1 to A4, B1 to B4, C1to C4 . . . are arranged in a line along the y-axis so as to be opposedto the peripheral surface of the corresponding photoreceptor drum 31.The quantity of light emitted from each of the light-emitting points A1to A4, B1 to B4, C1 to C4 . . . is relative to the drive current inputthereto. With respect to each of the light-emitting points A1 to A4, B1to B4, C1 to C4 . . . , the correlation between the quantity of emittedlight and the input drive current is, as shown by FIG. 2B, almostlinear.

The light-emitting points A1 to A4, B1 to B4, C1 to C4 . . . aresupported by one OLED substrate 52. Therefore, the light-emitting pointsA1 to A4, B1 to B4, C1 to C4 . . . can be produced in one process, andamong the light-emitting points A1 to A4, B1 to B4, C1 to C4 . . . ,there are substantially no variations in input-output characteristics.

Although each of the light-emitting point A1 to A4, B1 to B4, C1 to C4 .. . is a point light source, the light-emitting points A1 to A4, B1 toB4, C1 to C4 . . . as a whole are capable of scanning the peripheralsurface of the corresponding photoreceptor drum 31 with the light beamB.

The lens array 53 is held by the holder 51 to be opposed to thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . in thedirection of optical axes. The lens array 53 is, for example, a microlens array (MLA), a light collection transmitter array or the like. Eachof the micro lenses or the collection transmitters may be a rod lenswith plane end surfaces. Such components are easily processed, andtherefore, mass production of the OLED-PHs 17 is easy. The lens array 53focuses light B emitted from the light-emitting points A1 to A4, B1 toB4, C1 to C4 . . . on the peripheral surface of the correspondingphotoreceptor 31.

In the structure above, the peripheral surface of each of thephotoreceptor drums 31 can be scanned with a light beam B for thecorresponding color in a main-scanning direction (i.e., the y-axisdirection). Thereby, on the peripheral surface of each of thephotoreceptor drums 31, an electrostatic latent image for thecorresponding color is formed.

Although not shown in FIG. 2A for the sake of convenience, the OLED-PHs17 have cables, or the like, in addition to the shown components, forconnection to the other components of the image forming apparatus 1.

Again refer to FIG. 1. Each of the developing devices 35 has adeveloping roller extending in the y-axis direction. The developingroller is opposed to the peripheral surface of the correspondingphotoreceptor drum 31, immediately downstream of the exposure positionwhere the photoreceptor drum 31 is exposed to the beam B. In each of thedeveloping devices 35, for example, two-component developer of thecorresponding color is contained. Each of the developing devices 35supplies toner to the peripheral surface of the correspondingphotoreceptor drum 31 with the built-in developing roller. Thereby, theelectrostatic latent image on each of the photoreceptor drums 31 isdeveloped, and a toner image in the corresponding color (unicolor image)is formed.

Through the process above, each of the photoreceptor drums 31 supports atoner image on its peripheral surface. Also, while rotating in thedirection CW, each of the photoreceptor drums 31 carries the toner imagedownstream in the rotating direction CW.

Each of the transfer devices 19 extends in the y-axis direction and islocated immediately downstream of the developing device 35 for thecorresponding color. Each of the transfer devices 19 is opposed to theperipheral surface of the corresponding photoreceptor drum 31 via theintermediate transfer belt 21, which will be described below.

The intermediate transfer belt 21 is an endless belt. The intermediatetransfer belt 21 is stretched between the driving roller 23 and thedriven roller 25 so as to lie between the transfer devices 19 and thephotoreceptor drums 31. The intermediate transfer belt 21 is pressedonto the photoreceptor drums 31 by the transfer devices 19. The areaswhere the intermediate transfer belt 21 is in contact with thephotoreceptor drums 31 are referred to as primary transfer areas. Thedriving roller 23 is rotated by a drive force supplied from a motor (notshown). The driven roller 25 rotates following the rotation of thedriving roller 23. Thereby, the intermediate transfer belt 21 rotates inthe direction shown by arrow α.

A primary transfer bias voltage is applied to each of the transferdevices 19, and thereby, the areas where the transfer devices 19 are incontact with the intermediate transfer belt 21 are charged with anopposite polarity to the toner images. Accordingly, when the tonerimages carried by the photoreceptor drums 31 reach the primary transferareas, the toner images move to the outer surface of the intermediatetransfer belt 21. Thus, the toner images formed on the photoreceptordrums 31 are transferred to the intermediate transfer belt 21. In thefollowing, the transfer of the toner images to the intermediate transferbelt 21 is referred to as primary transfer.

At this stage, the toner images supported on the respectivephotoreceptor drums 31 are transferred sequentially on the same area ofthe intermediate transfer belt 21. By this primary transfer, tonerimages in the respective colors are combined, thereby resulting information of a composite toner image. The intermediate transfer belt 21supports the composite toner image on its outer surface, and carries thecomposite toner image to the secondary transfer roller 27 whilerotating.

The secondary transfer roller 27 is opposed to the driving roller 23 viathe intermediate transfer roller 21 and is pressed by the intermediatetransfer belt 21. The area where the intermediate transfer belt 21 is incontact with the secondary transfer roller 27 is hereinafter referred toas a secondary transfer area. As mentioned, the sheet S is fed to andpasses through the secondary transfer area, and the composite tonerimage supported on the intermediate transfer belt 21 is carried to thesecondary transfer area. A secondary transfer bias voltage is applied tothe secondary transfer roller 27, and thereby, the secondary transferroller 27, which is located at the back side (non-image-receiving side)of the sheet S, is charged with an opposite polarity to the compositetoner image. Accordingly, the composite toner image moves from the outersurface of the intermediate transfer belt 21 to the front side(image-receiving side) of the sheet S. Thus, the composite toner imagecarried by the intermediate transfer belt 21 is transferred to the sheetS. This image transfer to the sheet S is hereinafter referred to assecondary transfer.

The sheet S that has received the composite toner image is fed to thefixing device 9. The fixing device 9 fixes the composite toner image onthe sheet S by heating and pressing the sheet S. The sheet S that hasbeen subjected to the fixing process is ejected through the pair ofejection rollers 11 and is placed on the printed-sheet tray 13.

The respective sections described above are controlled by the controlcircuit 37 built in the body of the image forming apparatus 1. Thecontrol circuit 37 comprises a CPU, a main memory, etc. and runs inaccordance with a prepared program to control the printing operation ofthe image forming apparatus 1, and a gain setting operation, atemperature correction operation, and a light quantity correctionoperation, which will be described later.

Configuration of OLED-PH

Hereinafter, details of the OLED-PH 17 are described. FIG. 3 is a firstblock diagram showing details of the configuration of the OLED-PH 17 andsurrounding components. FIG. 4A is a second block diagram showingdetails of the configuration of the OLED-PH 17 and surroundingcomponents. FIG. 4B is a graph showing the output value (output current)of a photodetector 41 shown in FIG. 4A with respect to the quantity ofincident light. FIG. 5A is a circuit diagram showing details of theconfiguration of a photodetector 41A, an integrating circuit 42A, and aS/H circuit 43A shown in FIG. 3 and FIG. 4A. FIG. 5B is a circuitdiagram showing details of the configuration of a subtracting circuit 44shown in FIG. 3 and FIG. 4A. FIG. 5C is a vertical cross-sectional viewshowing details of the configuration of the photodetector 41A shown inFIG. 3 and FIG. 4A.

First, referring to FIG. 3 and FIG. 4A, the OLED-PH 17 includeslight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . and drivecircuits 6A1 to 6A4, 6B1 to 6B4, 6C1 to 6C4 . . . on an OLED substrate52. The light-emitting points A1 to A4, B1 to B4, C1 to C4 . . . aregrouped into a plurality of groups A, B, C . . . . The group A includes,for example, four light-emitting points A1 to A4. Likewise, the groupsB, C . . . include light-emitting points B1 to B4, C1 to C4 . . . ,respectively.

The drive circuit 6A1 is connected to the light-emitting point A1.Likewise, the drive circuits 6A2 to 6A4, 6B1 to 6B4, 6C1 to 6C4 . . .are connected to the light-emitting points A2 to A4, B1 to B4, C1 to C4. . . , respectively. Each of these drive circuits 6A1 to 6A4, 6B1 to6B4, 6C1 to 6C4 . . . includes a current source and a switch (switchingelement such as TFT). Note that only the current source and the switchof the drive circuits 6A1 are shown in the drawing.

The current sources of the drive circuits 6A1 to 6A4, 6B1 to 6B4, 6C1 to6C4 . . . are respectively supplied with current control signals ICSa1to ICSa4, ICSb1 to ICSb4, ICSc1 to ICSc4 . . . output from the controlcircuit 37. The current control signals ICSa1 to ICSa4, ICSb1 to ICSb4,ICSc1 to ICSc4 are driving signals for driving the light-emitting pointsA1 to A4, B1 to B4, C1 to C4 . . . , respectively. Each of the currentsources outputs an electric current of a value which is based on acorresponding current control signal.

The switches of the drive circuits 6A1 to 6A4, 6B1 to 6B4, 6C1 to 6C4 .. . are respectively supplied with switching signals ISa1 to ISa4, ISb1to ISb4, ISc1 to ISc4 . . . output from the control circuit 37. Theswitches are turned on/off based on corresponding switching signals. Theswitching signals are generated based on image signals which are to beprinted out by the image forming apparatus 1.

In this structure, for example, the current generated by the currentsource is supplied to the light-emitting point A1 while the currentsource of the drive circuit 6A1 generates a current based on the currentcontrol signal ICSa1 and the switch of the drive circuit 6A1 is kept onbased on the switching signal ISa1. Likewise, the other drive circuits6A2 to 6A4, 6B1 to 6B4, 6C1 to 6C4 . . . supply currents tocorresponding light-emitting points A2 to A4, B1 to B4, C1 to C4 . . .based on corresponding current control signals and switching signals.

The OLED substrate 52 is further provided with a set of components forthe group A, including a photodetector 41A, an integrating circuit 42A,and a S/H circuit 43A. Likewise, the OLED substrate 52 is furtherprovided with sets of components for the groups B, C . . . , including aset of a photodetector 41B, an integrating circuit 42B, and a S/Hcircuit 43B, a set of a photodetector 41C, an integrating circuit 42C,and a S/H circuit 43C, and so on.

Each of the photodetectors 41A, 41B, 41C . . . is realized by aphotodiode which is configured to detect the quantity of light radiatedat the light-emitting point A2 to A4, B1 to B4, C1 to C4 . . . of thecorresponding group A, B, C . . . , and which has a linear input-outputcharacteristic such as shown in FIG. 4B. The photodetectors 41A, 41B,41C . . . respectively receive and detect light emitted from thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . of the groupsA, B, C . . . , and output current values I_(PDA), I_(PDB), I_(PDC) . .. which depend on the quantity of received light to the integratingcircuits 42A, 42B, 42C . . . , respectively.

The integrating circuits 42A, 42B, 42C . . . respectively integrate theoutputs of the photodetectors 41A, 41B, 41C . . . of the previous stage.The S/H circuits 43A, 43B, 43C . . . respectively store the outputs ofthe integrating circuits 42A, 42B, 42C . . . of the previous stage.

The OLED substrate 52 is further provided with a subtracting circuit 44which is shared among the groups A, B, C . . . . The subtracting circuit44 carries out a subtracting operation using the outputs of the S/Hcircuits 43A, 43B, 43C . . . of the groups A, B, C . . . to outputphotodetection signals A, B, C . . . .

The control circuit 37 corrects the quantity of light emitted fromcorresponding light-emitting points A1 to A4, B1 to B4, C1 to C4 . . .based on the photodetection signals A, B, C . . . for respective ones ofthe light-emitting points which are obtained from the subtractingcircuit 44.

Next, details of the configuration of the groups A, B, C . . . shown inFIG. 3 and FIG. 4A are described. Note that the groups A, B, C . . .have the same configuration in details, and therefore, among others, theconfiguration of the group A is described in the following section.

Firstly, in FIG. 5A, the photodetector 41A is a thin-film photodiodesuch as shown in FIG. 5C. In FIG. 5C, a thin film transistor (TFT) whichis a constituent of the drive circuit 6A1, or the like, is provided onthe OLED substrate 52. This TFT is, more specifically, a LTPS-TFT(Low-Temperature Poly Silicon TFT). On the thus-configured OLEDsubstrate 52, a PIN photodiode (PIN-PD) in which amorphous silicon(a-Si) is used is formed by CVD (Chemical Vapor Deposition) as thephotodetector 41A. Further, on the OLED substrate 52, OLEDs which serveas light-emitting points A1 to A4, or the like, are formed bydeposition. In the present embodiment, the TFT and the PIN-PD arearranged so as not to overlap with the light-emitting surface of theOLED when viewed in plan in the normal direction of the light-emittingsurface of the OLED (i.e., light-emitting direction).

Again refer to FIG. 5A. The integrating circuit 42A includes anoperational amplifier 91, a plurality of capacitors CG1 to CG4, a switch92, and a switch 93 as shown in FIG. 5A. The integrating circuit 42Aintegrates the output of the photodetector 41A.

The inverted input terminal (−) of the operational amplifier 91 isconnected to the above-described photodetector 41A. One end of each ofthe capacitors CG1 to CG4 is connected to the inverted input terminal(−). The other end of each of the capacitors CG1 to CG4 is connected tothe switch 93.

The capacitors CG1 to CG4 are provided for the purpose of gainadjustment and have different capacitance values. In this embodiment,the capacitance values of the capacitors CG1 to CG4 are as follows:

CG1: 1 pF

CG2: 0.5 pF

CG3: 0.25 pF

CG4: 0.125 pF

The switch 93 is also connected to the output terminal of theoperational amplifier 91. The switch 93 selects any one of thecapacitors CG1 to CG4 based on a selection signal SELa supplied from thecontrol circuit 37 and connects the selected capacitor in parallelbetween the inverted input terminal (−) and the output terminal.

The switch 92 is located between the inverted input terminal (−) and theoutput terminal of the operational amplifier 91. The switch 92 makes orbreaks the connection between the inverted input terminal (−) and theoutput terminal according to a reset signal RSTa supplied from thecontrol circuit 37. When the switch 92 makes the connection, thevoltages accumulated in the capacitors CG1 to CG4 are reset to 0 V.

The non-inverted input terminal (+) of the operational amplifier 91 isgrounded.

The output terminal of the operational amplifier 91 is connected to theS/H circuit 43A. The output voltage V_(OUT) of the operational amplifier91 is calculated by expression (1) shown below:V _(OUT) =I _(PDA) ×T _(PHOTO) /C _(S)  (1)where C_(S) is the capacitance of the selected capacitor, I_(PDA) is theoutput current of the photodetector 41A, and T_(PHOTO) is the integraltime (the light-emitting time of the light-emitting point). The outputvoltage V_(OUT) is output as a photodetection signal A.

The other integrating circuits 42B, 42C . . . have the same structureand are controlled in the same way as the integrating circuit 42A, andoutput photodetection signals B, C . . . in accordance with the inputcurrents I_(PDB), I_(PDC) . . . , respectively.

In order to improve the detection accuracy of the photodetection signalsA, B, C . . . , it is necessary to raise the voltages V_(OUT) of thephotodetection signals A, B, C . . . such that a sufficient dynamicrange is ensured. However, for example, the quantity of light incidenton the photodetector 41A depends on the quantities of light emitted fromthe light-emitting points A1 to A4 and other factors. Even if thequantities of light emitted from the light-emitting points A1 to A4 areequal to one another, the current value I_(PDA) of the photodetector 41Amay be different for each of the light-emitting points A1 to A4.

Therefore, in this embodiment, prior to an ordinary optical writingprocess, a gain setting operation, which will be described later, iscarried out. In the gain setting operation, one of the capacitors CG1 toCG4 is selected in accordance with the quantity of light incident on thephotodetector 41A to set a gain fixedly for amplification of thephotodetection signal A in regard to each of the light-emitting pointsA1 to A4. Thereby, a sufficient dynamic range can be ensured. Withrespect to the other photodetectors 41B, 41C . . . , the same operationis carried out.

The S/H circuit 43A includes four S/H circuits 43A-1 to 43A-4. The S/Hcircuit 43A-1 includes a capacitor VR_(dark) which is configured to holdthe photodetection signal A of the integrating circuit 42A of theprevious stage, a voltage follower, and two switches, one preceding thevoltage follower and the other succeeding the voltage follower. The S/Hcircuit 43A-2 includes a capacitor VS_(dark) which is configured to holdthe photodetection signal A of the integrating circuit 42A of theprevious stage, a voltage follower, and two switches, one preceding thevoltage follower and the other succeeding the voltage follower. The S/Hcircuit 43A-3 includes a capacitor VR_(photo) which is configured tohold the photodetection signal A of the integrating circuit 42A of theprevious stage, a voltage follower, and two switches, one preceding thevoltage follower and the other succeeding the voltage follower. The S/Hcircuit 43A-4 includes a capacitor VS_(photo) which is configured tohold the photodetection signal A of the integrating circuit 42A of theprevious stage, a voltage follower, and two switches, one preceding thevoltage follower and the other succeeding the voltage follower. Here,the succeeding switches are arranged such that the values held by thecapacitors of the other groups are not simultaneously output to thesubtracting circuit 44 that is shared among the groups.

Next, refer to FIG. 5B. The subtracting circuit 44 is a two-stagedsubtracting circuit. The first stage includes a subtracting circuit 44-1and a subtracting circuit 44-2. The second stage includes a subtractingcircuit 44-3.

Although details of the process carried out by the subtracting circuits44-1 to 44-3 will be described later, the subtracting circuits 44-1,44-2 of the first stage are provided for the purpose of removing resetnoise that occurs in resetting the capacitors of the integrating circuit42A, and the subtracting circuit 44-3 of the second stage is providedfor the purpose of removing an dark output.

The OLED-PH 17 includes an ADC 46 and an averaging circuit 48. The ADC46 is provided at a succeeding stage of the subtracting circuit 44 andis configured to convert an analog output value of the subtractingcircuit 44-3 to a digital value.

The averaging circuit 48 averages a plurality of output values (in thepresent embodiment, eight output values) from the ADC 46. Specifically,the averaging circuit 48 includes, for example, eight registers. Theaveraging circuit 48 records the eight output values from the ADC 46 andadds these values together. After the addition, the averaging circuit 48divides the resultant value by eight for example, by removing thelow-order 3 bits. Note that the averaging circuit 48 may average atleast two output values. Although the averaging circuit 48 is shown aspart of the control circuit 37 in FIG. 4A, the averaging circuit 48 maybe provided outside the control circuit 37.

In the OLED-PH 17 that has the above-described configuration, in anordinary optical writing process, control signals. (e.g., horizontalsynchronization signals, clock signals, etc.) and image data aretransmitted from the control circuit 37 to the OLED-PH 17. In theOLED-PH 17, the drive circuits 6A1 to 6A4, 6B1 to 6B4, 6C1 to 6C4 . . .control the lighting-on (ON) and lighting-off (OFF) of thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . based on theinput image data. As a result, an electrostatic latent image is formedon the peripheral surface of each of the charged photoreceptor drums 31(see FIG. 2A).

The quantity of light (i.e., the intensity of light) emitted from thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . is controlledprior to the optical writing process. For example, when the imageforming apparatus 1 is powered on, light quantity set values aretransferred from a memory section 50, which is realized by anon-volatile memory, for example, to storage sections (e.g., registers)provided in the drive circuits 6A1 to 6A4, 6B1 to 6B4, 6C1 to 6C4 . . ., and the transferred values are stored in the storage sections. Basedon these light quantity set values, each of the light-emitting points A1to A4, B1 to B4, C1 to C4 . . . emits light of a desired light emissionquantity. Note that although the memory section 50 is shown as beingprovided outside the control circuit 37, the memory section 50 may beprovided inside the control circuit 37.

Control of the quantity of light emitted from the light-emitting pointsA1 to A4, B1 to B4, C1 to C4 . . . requires light quantity correction.Operations of light quantity detection and correction which are to becarried out to this end will be described later.

In the OLED-PH 17, the photodetectors 41A, 41B, 41C . . . have atemperature dependence. Further, the integrating circuits 42A, 42B, 42C. . . and the S/H circuits 43A, 43B, 43C . . . , which are readoutcircuits, also have a temperature dependence as the photodetectors 41A,41B, 41C do. Specifically, as the temperatures of the photodetectors41A, 41B, 41C . . . , the integrating circuits 42A, 42B, 42C . . . , andthe S/H circuits 43A, 43B, 43C increase, the output values from thesecomponents are greater than those corresponding to the emitted lightquantities actually detected by the photodetectors 41A, 41B, 41C . . . .On the other hand, as the temperatures of the photodetectors 41A, 41B,41C . . . , the integrating circuits 42A, 42B, 42C . . . , and the S/Hcircuits 43A, 43B, 43C decrease, the output values from these componentsare smaller than those corresponding to the emitted light quantitiesactually detected by the photodetectors 41A, 41B, 41C . . . . In view ofsuch, in the OLED-PH 17, the temperatures of the photodetectors 41A,41B, 41C . . . are determined, and the photodetection signals A, B, C .. . are corrected based on the temperatures. In the following section,such an operation is referred to as “temperature correction operation”.

In the temperature correction operation, the control circuit 37determines the temperatures of the photodetectors 41A, 41B, 41C . . .based on the photodetection signals A, B, C . . . which are output fromthe integrating circuits 42A, 42B, 42C . . . while the light-emittingpoints A1 to A4, B1 to B4, C1 to C4 . . . are OFF. Hereinafter,photodetection signals A, B, C . . . which are output from theintegrating circuits 42A, 42B, 42C . . . while the light-emitting pointsA1 to A4, B1 to B4, C1 to C4 . . . are OFF are generically referred toas “dark-time output signals”. The S/H circuits 43A, 43B, 43C . . . ,the subtracting circuit 44, and the averaging circuit 48 extractphotodiode dark currents from the dark-time output signals. Details ofthe temperature correction operation will be described later.

General Procedure of Operation of OLED-PH

Next, the operation of the OLED-PH 17 is described with reference to thedrawings. FIG. 6 is a flowchart of an operation carried out by thecontrol circuit 37 of the OLED-PH 17.

This process is started when the image forming apparatus 1 is poweredon. The control circuit 37 determines whether or not the temperaturecorrection operation is to be carried out (step S01). At step S01,specifically, the control circuit 37 determines whether or not apredetermined time period has passed since the last temperaturecorrection operation. The predetermined time period is, for example,several tens of minutes. Note that, however, the predetermined timeperiod is not limited to several tens of minutes. Note that, immediatelyafter being powered on, the control circuit 37 determines that thetemperature correction operation is to be carried out even if thepredetermined time period has not passed yet. If the temperaturecorrection operation is to be carried out, the process proceeds to stepS02. If the temperature correction operation is not to be carried out,the process proceeds to step S03.

If the temperature correction operation is to be carried out, thecontrol circuit 37 carries out a temperature correction operation whichwill be described later. When the temperature correction operation isfinished, the process returns to step S01.

If the temperature correction operation is not to be carried out, thecontrol circuit 37 determines whether or not the gain setting operationis to be carried out (step S03). At step S03, specifically, the controlcircuit 37 determines whether or not a predetermined time period haspassed since the last gain setting operation. The predetermined timeperiod is, for example, twelve hours. Note that, however, thepredetermined time period is not limited to twelve hours. Note that,immediately after being powered on, the control circuit 37 determinesthat the gain setting operation is to be carried out even if thepredetermined time period has not passed yet. If the gain settingoperation is to be carried out, the process proceeds to step S04. If thegain setting operation is not to be carried out, the process proceeds tostep S05.

If the gain setting operation is to be carried out, the control circuit37 carries out a gain setting operation which will be described later(step S04). When the gain setting operation is finished, the processreturns to step S01.

If the gain setting operation is not to be carried out, the controlcircuit 37 determines whether or not the light quantity correctionoperation is to be carried out (step S05). At step S05, specifically,the control circuit 37 determines whether or not a predetermined timeperiod has passed since the last light quantity correction operation.The predetermined time period is, for example, several tens of minutes.Note that, however, the predetermined time period is not limited toseveral tens of minutes. Note that, immediately after being powered on,the control circuit 37 determines that the light quantity correctionoperation is to be carried out even if the predetermined time period hasnot passed yet. If the light quantity correction operation is to becarried out, the process proceeds to step S06. If the light quantitycorrection operation is not to be carried out, the process proceeds tostep S07.

If the light quantity correction operation is to be carried out, thecontrol circuit 37 carries out a light quantity correction operationwhich will be described later (step S06). When the light quantitycorrection operation is finished, the process returns to step S01.

If the light quantity correction operation is not to be carried out, thecontrol circuit 37 determines whether or not the process is to be ended(step S07). At step S07, specifically, the control circuit 37determines, for example, whether or not the image forming apparatus 1 ispowered off. If the process is not to be ended, the process returns tostep S01. The operations of step S01 through step S07 are repeated tillthe image forming apparatus 1 is powered off.

Temperature Correction Operation

Next, the temperature correction operation of step S02 is described withreference to the drawings. FIG. 7 is a timing chart of the temperaturecorrection operation.

In the temperature correction operation of the present embodiment, thecontrol circuit 37 determines the temperatures of the photodetectors41A, 41B, 41C . . . based on the magnitude of photodiode dark currentsincluded in the current values I_(PDA), I_(PDB), I_(PDC) . . . which areoutput from the photodetectors 41A, 41B, 41C . . . while thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . are OFF.Specifically, in the temperature correction operation, the OLED-PH 17allows the integrating circuits 42A, 42B, 42C . . . to respectivelyintegrate the current values I_(PDA), I_(PDB), I_(PDC) . . . which areoutput from the photodetectors 41A, 41B, 41C . . . while thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . are OFF. Then,the OLED-PH 17 extracts photodiode dark currents from the photodetectionsignals A, B, C (dark-time output signals) output from the integratingcircuits 42A, 42B, 42C . . . . Therefore, it is necessary to removenoise, exclusive of the photodiode dark currents, from the dark-timeoutput signals. The noise components included in the dark-time outputsignals are shown below.

(1) Dark Current Shot Noise

Dark current shot noise refers to noise which is attributed to a darkcurrent produced in a photodiode and which is an irregularly fluctuatingcomponent of the dark current. Thus, the dark current shot noise israndom noise.

(2) Reset Noise

Reset noise refers to noise which occurs after a voltage is applied to acapacitance via a switch and the switch is turned off and which isproduced in a sampling circuit or the like. The reset noise is randomnoise.

(3) Amplification Noise

Amplification noise refers to noise which is produced in anamplification circuit. The amplification noise is random noise.

(4) Optical Shot Noise

Optical shot noise refers to noise which occurs because light has theproperties of a particle called photon so that, even if the lightintensity is constant, the number of photons of light incident on aphotodiode within one cycle of the accumulation time is not always thesame but fluctuates. Thus, the optical shot noise is random noise.

(5) Photodiode Dark Current

Photodiode dark current refers to noise which is attributed to a darkcurrent variation in each pixel and which has such a characteristic thatit has a temperature dependence while the noise signal voltage isproportional to the accumulation time. The photodiode dark current isfixed pattern noise.

(6) Photodiode Sensitivity Variation

Photodiode sensitivity variation refers to noise which is produced dueto the variation of the sensitivity of a photodiode. The photodiodesensitivity variation is fixed pattern noise.

The optical shot noise and the photodiode sensitivity variation are tobe produced when there is light incident on the photodetectors 41A to41C. The dark current shot noise, reset noise, amplification noise, andphotodiode dark current are to be produced both when there is lightincident on the photodetectors 41A to 41C and when there is no lightincident on the photodetectors 41A to 41C. In the temperature correctionoperation, the temperatures of the photodetectors 41A, 41B, 41C . . .are determined based on the dark-time output signals. Therefore, theoptical shot noise and the photodiode sensitivity variation arenegligible. The dark current shot noise is much smaller than thephotodiode dark current and is also negligible.

As understood from the foregoing, the photodiode dark current can beextracted by removing the reset noise and the amplification noise fromthe dark-time output signal. Hereinafter, the temperature correctionoperation is described in more detail.

Prior to the start of the temperature correction operation, the controlcircuit 37 sends the selection signal SELa to the switch 93 to selectthe capacitor CG4 that has the smallest capacitance. Since the dark-timeoutput signal is very small, improving the detection accuracy of thedark-time output signal requires further increasing the voltage valueV_(OUT) of the dark-time output signal such that a sufficient dynamicrange is ensured. Therefore, the control circuit 37 selects thecapacitor CG4 for the purpose of gain setting which is suitable to asignal level of 0 V to 0.25 V.

In the temperature correction operation, the control circuit 37 obtainsmore than once the output value V_(sig) that has been obtained byremoving the reset noise from a dark-time output signal. In the presentembodiment, the control circuit 37 obtains eight output values V_(sig).

First, the control circuit 37 obtains the first output value V_(sig) atstep S21 through step S23. Specifically, at step S21, the controlcircuit 37 does not allow emission of the light-emitting points A1 toA4. Thereafter, the control circuit 37 sends the control signal RSTa tocancel the reset state of the integrating circuit 42A so that the outputof the photodetector 41A can be accumulated in the capacitor CG4 of theintegrating circuit 42A. At the same time, the control circuit 37 sendsa control signal SHR_(photo) to close the switch on the preceding stageside of the S/H circuit 43A-3 so that the output value of theintegrating circuit 42A (at the start of integration) is recorded in thecapacitor VR_(photo); the control circuit 37 sends a control signalSHR_(dark) to close the switch on the preceding stage side of the S/Hcircuit 43A-1 so that the output value of the integrating circuit 42A(at the start of integration) is recorded in the capacitor VR_(dark);and the control circuit 37 sends a control signal SHS_(dark) to closethe switch on the preceding stage side of the S/H circuit 43A-2 so thatthe output value of the integrating circuit 42A (at the start ofintegration) is recorded in the capacitor VS_(dark). As a result,voltage value V1 is applied to the capacitors VR_(photo), VR_(dark),VS_(dark). Thereafter, the control circuit 37 sends the control signalSHR_(photo) to open the switch on the preceding stage side of the S/Hcircuit 43A-3, sends the control signal SHR_(dark) to open the switch onthe preceding stage side of the S/H circuit 43A-1, and sends the controlsignal SHS_(dark) to open the switch on the preceding stage side of theS/H circuit 43A-2.

At the subsequent step S22, after a predetermined time period has passedsince the start of integration (i.e., after passage of the integraltime) while the light-emitting points A1 to A4 are OFF, the controlcircuit 37 sends a control signal SHS_(photo) to close the switch on thepreceding stage side of the S/H circuit 43A-4 and records the outputvalue of the integrating circuit 42A (after passage of the integraltime) in the capacitor VS_(photo). As a result, voltage value V2 isapplied to the capacitor VS_(photo). Thereafter, the control circuit 37sends the control signal SHS_(photo) to open the switch on the precedingstage side of the S/H circuit 43A-4.

At the subsequent step S23, the control circuit 37 sends a controlsignal SEL to close the switches on the succeeding stage side of the S/Hcircuits 43A-1 to 43A-4. Accordingly, the subtracting circuit 44-2subtracts voltage value V1 held by the capacitor VR_(photo) from voltagevalue V2 held by the capacitor VS_(photo). Here, voltage value V1 is anoutput value of the integrating circuit 42A which is obtained at thestart of integration while the light-emitting points A1 to A4 are OFF,and is formed by the reset noise. Voltage value V2 is an output value ofthe integrating circuit 42A which is obtained after the integral timehas passed since the start of integration while the light-emittingpoints A1 to A4 are OFF, and is formed by the reset noise, theamplification noise, and the photodiode dark current. Thus, by the abovesubtraction, an output value V_(photo) of the photodetector 41A fromwhich the reset noise has been removed is obtained. The output valueV_(photo) is output to the subtracting circuit 44-3 of the succeedingstage. The output value V_(photo) is voltage value V5 (=V2-V1).

Further, the subtracting circuit 44-1 subtracts voltage value V1 held bythe capacitor VR_(dark) from voltage value V1 held by the capacitorVS_(dark). Therefore, as a result of this subtraction, the output valueV_(dark) of the subtracting circuit 44-1 is zero (0). This result isoutput to the subtracting circuit 44-3 of the succeeding stage.

The subtracting circuit 44-3 subtracts the output value V_(dark)(voltage value 0) from the output value V_(photo) (voltage value V5) andoutputs the output value V_(sig) (voltage value V5) to the ADC 46 of thesucceeding stage. The ADC 46 outputs the output value V_(sig) (voltagevalue V5) to the control circuit 37. As a result, the control circuit 37obtains the first output value V_(sig). The control circuit 37 writesthe first output value V_(sig) in the first register of the averagingcircuit 48. Then, the control circuit 37 sends the control signal RSTato reset the integrating circuit 42A.

Then, the control circuit 37 obtains the second output value V_(sig) instep S24 through step S26. In step S24 through step S26, the controlcircuit 37 carries out the same operations as those of step S21 throughstep S23. Specifically, at step S24, the control circuit 37 does notallow emission of the light-emitting points A1 to A4. Thereafter, thecontrol circuit 37 sends the control signal RSTa to cancel the resetstate of the integrating circuit 42A so that the output of thephotodetector 41A can be accumulated in the capacitor CG4 of theintegrating circuit 42A. At the same time, the control circuit 37 sendsthe control signal SHR_(photo) to close the switch on the precedingstage side of the S/H circuit 43A-3 so that the output value of theintegrating circuit 42A (at the start of integration) is recorded in thecapacitor VR_(photo); the control circuit 37 sends the control signalSHR_(dark) to close the switch on the preceding stage side of the S/Hcircuit 43A-1 so that the output value of the integrating circuit 42A(at the start of integration) is recorded in the capacitor VR_(dark);and the control circuit 37 sends the control signal SHS_(dark) to closethe switch on the preceding stage side of the S/H circuit 43A-2 so thatthe output value of the integrating circuit 42A (at the start ofintegration) is recorded in the capacitor VS_(dark). As a result,voltage value V3 is applied to the capacitors VR_(photo), VR_(dark),VS_(dark). Thereafter, the control circuit 37 sends the control signalSHR_(photo) to open the switch on the preceding stage side of the S/Hcircuit 43A-3, sends the control signal SHR_(dark) to open the switch onthe preceding stage side of the S/H circuit 43A-1, and sends the controlsignal SHS_(dark) to open the switch on the preceding stage side of theS/H circuit 43A-2.

At the subsequent step S25, after a predetermined time period has passedsince the start of integration (i.e., after passage of the integraltime) while the light-emitting points A1 to A4 are OFF, the controlcircuit 37 sends the control signal SHS_(photo) to close the switch onthe preceding stage side of the S/H circuit 43A-4 and records the outputvalue of the integrating circuit 42A (after passage of the integraltime) in the capacitor VS_(photo). As a result, voltage value V4 isapplied to the capacitor VS_(photo). Thereafter, the control circuit 37sends the control signal SHS_(photo) to open the switch on the precedingstage side of the S/H circuit 43A-4.

At the subsequent step S26, the control circuit 37 sends the controlsignal SEL to close the switches on the succeeding stage side of the S/Hcircuits 43A-1 to 43A-4. Accordingly, the subtracting circuit 44-2subtracts voltage value V3 held by the capacitor VR_(photo) from voltagevalue V4 held by the capacitor VS_(photo). Here, voltage value V4 is anoutput value of the integrating circuit 42A which is obtained at thestart of integration while the light-emitting points A1 to A4 are OFF,and is formed by the reset noise. Voltage value V3 is an output value ofthe integrating circuit 42A which is obtained after the integral timehas passed since the start of integration while the light-emittingpoints A1 to A4 are OFF, and is formed by the reset noise, theamplification noise, and the photodiode dark current. Thus, by the abovesubtraction, an output value V_(photo) of the photodetector 41A fromwhich the reset noise has been removed is obtained. The output valueV_(photo) is output to the subtracting circuit 44-3 of the succeedingstage. The output value V_(photo) is voltage value V6 (=V4-V3).

Further, the subtracting circuit 44-1 subtracts voltage value V3 held bythe capacitor VR_(dark) from voltage value V3 held by the capacitorVS_(dark). Therefore, as a result of this subtraction, the output valueV_(dark) of the subtracting circuit 44-1 is zero (0). This result isoutput to the subtracting circuit 44-3 of the succeeding stage.

The subtracting circuit 44-3 subtracts the output value V_(dark)(voltage value 0) from the output value V_(photo) (voltage value V6) andoutputs the output value V_(sig) (voltage value V6) to the ADC 46 of thesucceeding stage. The ADC 46 outputs the output value V_(sig) (voltagevalue V6) to the control circuit 37. As a result, the control circuit 37obtains the second output value V_(sig). The control circuit 37 writesthe second output value V_(sig) in the second register of the averagingcircuit 48. Then, the control circuit 37 sends the control signal RSTato reset the integrating circuit 42A.

Thereafter, the control circuit 37 repeats six times the same operationsas those of step S21 through step S23 or step S24 through step S26 toobtain the third through eighth output values V_(sig).

Here, the first through eighth output values V_(sig) include, inaddition to the photodiode dark current, the amplification noise that israndom noise which randomly varies to positive or negative values.Therefore, as shown in FIG. 7, the difference between voltage value V5(first output value V_(sig)) and voltage value V6 (second output valueV_(sig)) is Δ V. The averaging circuit 48 outputs the average V_(ave) ofthe first through eighth output values V_(sig) to the control circuit37. Specifically, the averaging circuit 48 adds together the outputvalues V_(sig) recorded in the first through eighth registers, anddivides the resultant value by eight for example, by removing thelow-order 3 bits, thereby obtaining the average V_(ave). By averaging,the amplification noise, which is random noise, is removed from thefirst through eighth output values V_(sig). The control circuit 37accepts the obtained average V_(ave) as the magnitude of the photodiodedark current.

Then, the control circuit 37 determines the temperatures of thephotodetectors 41A, 41B, 41C . . . based on the magnitude of thephotodiode dark current (average V_(ave)). To this end, a table as shownby Table 1 below is preliminarily stored in the memory section 50.

TABLE 1 (d) PD Dark Current (PDDC) Temperature 8.0 mV ≤ PDDC 50° C. 6.96mV ≤ PDDC < 8.0 mV 48° C. 6.06 mV ≤ PDDC < 6.96 mV 46° C. 5.28 mV ≤ PDDC< 6.06 mV 44° C. 4.59 mV ≤ PDDC < 5.28 mV 42° C. 4.0 mV ≤ PDDC < 4.59 mV40° C. 3.48 mV ≤ PDDC < 4.0 mV 38° C. 3.03 mV ≤ PDDC < 3.48 mV 36° C.2.64 mV ≤ PDDC < 3.03 mV 34° C. 2.3 mV ≤ PDDC < 2.64 mV 32° C. 2.0 mV ≤PDDC < 2.3 mV 30° C. 1.74 mV ≤ PDDC < 2.0 mV 28° C. 1.52 mV ≤ PDDC <1.74 mV 26° C. 1.32 mV ≤ PDDC < 1.52 mV 24° C. 1.15 mV ≤ PDDC < 1.32 mV22° C. 1.0 mV ≤ PDDC < 1.15 mV 20° C. 0.87 mV ≤ PDDC < 1.0 mV 18° C.0.76 mV ≤ PDDC < 0.87 mV 16° C. 0.66 mV ≤ PDDC < 0.76 mV 14° C. 0.57 mV≤ PDDC < 0.66 mV 12° C. 0.5 mV ≤ PDDC < 0.57 mV 10° C. 0.44 mV ≤ PDDC <0.5 mV  8° C. 0.38 mV ≤ PDDC < 0.44 mV  6° C. 0.33 mV ≤ PDDC < 0.38 mV 4° C. 0.29 mV ≤ PDDC < 0.33 mV  2° C. 0.25 mV ≤ PDDC < 0.29 mV  0° C.

The table shows the relationship between the magnitude of the photodiodedark current and the temperatures of the photodetectors 41A, 41B, 41C .. . . As previously described, the photodiode dark current has thetemperature dependence. Specifically, the photodiode dark currentincreases as the temperature increases. Therefore, the table showshigher temperatures as the magnitude of the photodiode dark currentincreases.

The control circuit 37 refers to Table 1 to determine the temperature ofthe photodetector 41A based on the magnitude of the photodiode darkcurrent (average V_(ave)). For example, when the magnitude of thephotodiode dark current (average V_(ave)) is 2.2 mV, the control circuit37 determines that the temperature of the photodetector 41A is 30° C.,which corresponds to the range of “2. mV≤PDDC<2.3 mV”. Note that thecontrol circuit 37 may determine the temperature of the photodetector41A by an operation based on an interpolation function (interpolationoperation), rather than using the table.

In a light quantity correction operation which will be described later,the control circuit 37 corrects the photodetection signal A based on thetemperature of the photodetector 41A. To this end, a table as shown byTable 2 below is preliminarily stored in the memory section 50.

TABLE 2 Temperature Coefficient 50° C. 0.85 48° C. 0.86 46° C. 0.87 44°C. 0.88 42° C. 0.89 40° C. 0.90 38° C. 0.91 36° C. 0.92 34° C. 0.93 32°C. 0.94 30° C. 0.95 28° C. 0.96 26° C. 0.97 24° C. 0.98 22° C. 0.99 20°C. 1.00 18° C. 1.01 16° C. 1.02 14° C. 1.03 12° C. 1.04 10° C. 1.05  8°C. 1.06  6° C. 1.07  4° C. 1.08  2° C. 1.09  0° C. 1.10

The table shows the relationship between the temperatures of thephotodetectors 41A, 41B, 41C . . . and the temperature correctioncoefficients. The temperature correction coefficients are provided forrespective one of the temperatures of the photodetectors 41A, 41B, 41C .. . . The photodetection signals A, B, C . . . which are detected in thelight quantity correction operation which will be described later aremultiplied by the temperature correction coefficients corresponding tothe temperatures of the photodetectors 41A, 41B, 41C . . . , wherebycorrected photodetection signals A, B, C . . . are obtained. Thestrength of the photodetection signals A, B, C . . . which have passedthrough the integrating circuits 42A, 42B, 42C . . . and the S/Hcircuits 43A, 43B, 43C . . . increases as the temperatures of thephotodetectors 41A, 41B, 41C . . . , the integrating circuits 42A, 42B,42C . . . , and the S/H circuits 43A, 43B, 43C . . . increase.Therefore, the table shows smaller coefficients as the temperature ofthe photodetectors 41A, 41B, 41C . . . increases.

The control circuit 37 refers to Table 2 to determine the temperaturecorrection coefficient based on the temperature of the photodetector41A. For example, when the temperature of the photodetector 41A is 30°C., the control circuit 37 determines that the temperature correctioncoefficient is 0.95. The control circuit 37 stores the determinedtemperature correction coefficient in the memory section 50. The controlcircuit 37 uses the temperature correction coefficient stored in thememory section 50 in the temperature correction operation which will bedescribed later.

The control circuit 37 also carries out the above-described operationson the photodetectors 41B, 41C . . . .

Gain Setting Operation

Next, the gain setting operation of step S04 is described with referenceto the drawings. FIG. 8 is a subroutine of the gain setting operation ofstep S02 of FIG. 6. FIG. 9 is a timing chart of the gain settingoperation. Although FIG. 8 shows only the gain setting operation carriedout on the group A, the same gain setting operation is carried out onthe other groups.

In FIG. 8, firstly, the initial conditions are set (step S11).Specifically, the control circuit 37 sends the selection signal SELa tothe switch 93 to select one of the capacitors CG1 to CG4. In the presentembodiment, the capacitor CG1 is selected and connected between theinverted input terminal (−) and the output terminal of the operationalamplifier 91. Here, in setting the initial conditions, an OLED-PH 17whose cumulative light-emitting time is almost zero (i.e., unusedOLED-PH 17) is used.

The control circuit 37 further sets a common drive current value to thelight-emitting points A1 to A4 by the current control signals ICSa1 toICSa4. In the present embodiment, the drive current value is 5 μA.

Then, the control circuit 37 selects one of the light-emitting points A1to A4 as the first target of the process (step S12).

Then, the control circuit 37 sends the reset signal RSTa to open theswitch 92 so that the capacitor CG1 is chargeable (step S13).

Then, the control circuit 37 sends the switching signal ISa to the drivecircuit 6A connected to the targeted light-emitting point to turn on thedrive circuit 6A. The drive circuit 6A is kept on for a predeterminedtime period (e.g., 1 ms) to allow the targeted light-emitting point A toemit light (step S14). The predetermined time period is a charge time(i.e., integral time) of the capacitor CG1.

The photodetector 41A receives the light emitted from the light-emittingpoint A and outputs a current value I_(PDA) in accordance with thequantity of the received light. The current value I_(PDA) is sent to thegain switch circuit 9A, whereby the capacitor CG1 is charged for aperiod of 1 ms. In this period, the gain switch circuit 9A outputs aphotodetection signal A which is relative to the integral of inputvoltages.

Then, the control circuit 37 first receives the photodetection signal Ain regard to the targeted light-emitting point, the photodetectionsignal A being relative to the integral value for which the capacitorCG1 was used, and detects the voltage value of the photodetection signalA (step S15).

A table as shown by Table 3 below is preliminarily stored in the memorysection 50.

TABLE 3 V_(OUT) of Photodetection Signal A Capacitor to be Selected 0 V≤ V_(OUT) < 0.25 V CG4: 0.125 pF 0.25 V ≤ V_(OUT) < 0.5 V CG3: 0.25 pF0.5 V ≤ V_(OUT) < 1 V CG2: 0.5 pF 1 V ≤ V_(OUT) < 2 V CG1: 1 pF

The table shows a capacitor to be selected (one of the capacitors CG1 toCG4) for each voltage range of the photodetection signal A. In order toensure a wide dynamic range, as is apparent from the expression (1), itis necessary that the output voltage V_(OUT) in the process of ordinaryoptical writing is high. Therefore, the table shows a capacitor with asmaller capacitance for a lower voltage V_(OUT) of the photodetectionsignal A. For example, Table 3 above shows the followings: when thevoltage V_(OUT) is equal to or more than 0 V and less than 0.25 V, thecapacitor CG4 (0.125 pF) is selected; when the voltage V_(OUT) is equalto or more than 0.25 V and less than 0.5 V, the capacitor CG3 (0.25 pF)is selected; when the voltage V_(OUT) is equal to or more than 0.5 V andless than 1.0 V, the capacitor CG2 (0.5 pF) is selected; and when thevoltage V_(OUT) is equal to or more than 1.0 V and less than 2.0 V, thecapacitor CG4 (1 pF) is selected.

When the control circuit 37 detects a voltage of the photodetectionsignal A in regard to the targeted light-emitting point, the controlcircuit 37 selects one of the capacitors CG1 to CG4 with reference tothe table (step S15).

Then, the control circuit 37 sends the reset signal RSTa to close theswitch 92 so that the capacitor CG1 is discharged to 0 V (step S16).

Then, the control circuit 37 determines whether or not all thelight-emitting points A1 to A4 of the group A have been selected (stepS17). If all the light-emitting points A1 to A4 have not been selected,the process proceeds to step S18. If all the light-emitting points A1 toA4 have been selected, the process ends.

If all the light-emitting points A1 to A4 have not been selected, thecontrol circuit 37 selects one of unselected light-emitting points ofthe group A (step S18). Thereafter, the process returns to step S13.Then, the operations of steps S13 to S17 are carried out on the newtarget.

Next, a specific example of the gain setting operation is described withreference to the timing chart of FIG. 9.

First, at step S11 of setting the initial conditions, the controlcircuit 37 sends the current control signals ICSa1 to ICSa4 to set acommon drive current value (in this embodiment, 5 μA) to thelight-emitting points A1 to A4. The drive current value is such a valueto allow the OLED-PH 17 at the initial stage of use (that is, beforedeterioration) to emit an adequate quantity of light to thephotoreceptor drum 31.

Thereafter, at the first time of coming to step S14, the control circuit37 outputs the switching signal ISa1 for the first target light-emittingpoint A so as to allow the light-emitting point A to emit light for 1ms.

The current value I_(PDA-A1) from the photodetector 41A while thelight-emitting point A1 is selected is assumed to be 0.4 nA. The chargetime (in other words, the integral time of the gain switch circuit 9A)T_(PHOTO) is 1 ms. Under these conditions, the voltage value of thephotodetection signal A while the light-emitting point A1 is selected,V_(OUT-A1), is calculated as shown by the following expression (2).V _(OUT-A1)=0.4nA×1ms/1pF=0.4V  (2)

At the first time of coming to step S15, the table is referred to selectone of the capacitors CG1 to CG4 as the capacitor to be used forordinary optical writing. In regard to the light-emitting point A1,since the voltage V_(OUT-A1) is 0.4 V, the capacitor CG3 is selected asshown in Table 3.

Thereafter, at step S18, for example, the light-emitting point A2 isselected as the next target. In this case, at the second time of comingto step S14, the switching signal ISa2 is output, whereby thelight-emitting point A2 emits light for 1 ms.

While the light-emitting point A2 is selected, the current valueI_(PDA-A2) is assumed to be 0.8 nA. The charge time T_(PHOTO) is 1 ms.The voltage value of the photodetection signal A while thelight-emitting point A2 is selected, V_(OUT-A2), is calculated as shownby the following expression (3).V _(OUT-A2)=0.8nA×1ms/1pF=0.8V  (3)

At the second time of coming to step S15, in regard to thelight-emitting point A2, the capacitor CG2 is selected as the capacitorto be used for ordinary optical writing as shown in Table 3.

Thereafter, at step S18, for example, the light-emitting point A3 isselected as the next target. In this case, at the third time of comingto step S14, the switching signal ISa3 is output, whereby thelight-emitting point A3 emits light for 1 ms.

The current value I_(PDA-A3) from the photodetector 41A while thelight-emitting point A3 is selected is assumed to be 1.6 nA. The chargetime T_(PHOTO) is 1 ms. The voltage value of the photodetection signal Awhile the light-emitting point A3 is selected, V_(OUT-A3), is calculatedas shown by the following expression (4).V _(OUT-A3)=1.6nA×1ms/1pF=1.6V  (4)

At the third time of coming to step S15, in regard to the light-emittingpoint A3, the capacitor CG1 is selected as the capacitor to be used forordinary optical writing as shown in Table 3.

Thereafter, at step S18, for example, the light-emitting point A4 isselected as the next target. In this case, at the fourth time of comingto step S14, the switching signal ISa4 is output, whereby thelight-emitting point A4 emits light for 1 ms.

The current value I_(PDA-A4) from the photodetector 41A while thelight-emitting point A4 is selected is assumed to be 1.2 nA. The chargetime T_(PHOTO) is 1 ms. The voltage of the photodetection signal A whilethe light-emitting point A4 is selected, V_(OUT-A4), is calculated asshown by the following expression (5).V _(OUT-A4)=1.2nA×1ms/1pF=1.2V  (5)

At the fourth time of coming to step S15, with respect to thelight-emitting point A4, the capacitor CG1 is selected as the capacitorto be used for ordinary optical writing as shown in Table 3.

Light Quantity Correction Operation

Next, the light quantity correction operation of step S06 is describedwith reference to the drawings. FIG. 10 is a timing chart of the lightquantity correction operation. FIG. 11 is a timing chart showing aprocedure of light quantity detection on a group-by-group basis.

First, at step S31, the control circuit 37 allows only any one of thelight-emitting points A1 to A4 which is the target of light quantitydetection (e.g., light-emitting point A1) via corresponding one of thedrive circuits 6A1 to 6A4 (e.g., drive circuit 6A1). Thereafter, thecontrol circuit 37 sends the control signal RSTa to cancel the resetstate of the integrating circuit 42A so that the output of thephotodetector 41A can be accumulated in the capacitor of the integratingcircuit 42A. At the same time, the control circuit 37 sends the controlsignal SHR_(photo) to close the switch on the preceding stage side ofthe S/H circuit 43A-3 so that the output value of the integratingcircuit 42A (at the start of integration) is recorded in the capacitorVR_(photo). Thereafter, the control circuit 37 sends the control signalSHR_(photo) to open the switch on the preceding stage side of the S/Hcircuit 43A-3.

At the subsequent step S32, after a predetermined time period has passedsince the start of integration (i.e., after passage of the integraltime) while only any one of the light-emitting points A1 to A4 which isthe target of light quantity detection (e.g., light-emitting point A1)is kept ON, the control circuit 37 sends the control signal SHS_(photo)to close the switch on the preceding stage side of the S/H circuit 43A-4and records the output value of the integrating circuit 42A (afterpassage of the integral time) in the capacitor VS_(photo). Thereafter,the control circuit 37 sends the control signal SHS_(photo) to open theswitch on the preceding stage side of the S/H circuit 43A-4 and sendsthe control signal RSTa to reset the integrating circuit 42A.

At the subsequent step S33, the control circuit 37 turns off all thelight-emitting points A1 to A4. Thereafter, the control circuit 37 sendsthe control signal RSTa to cancel the reset state of the integratingcircuit 42A so that the output of the photodetector 41A can beaccumulated in the capacitor of the integrating circuit 42A. At the sametime, the control circuit 37 sends the control signal SHR_(dark) toclose the switch on the preceding stage side of the S/H circuit 43A-1 sothat the output value of the integrating circuit 42A (at the start ofintegration) is recorded in the capacitor VR_(dark). Thereafter, thecontrol circuit 37 sends the control signal SHR_(dark) to open theswitch on the preceding stage side of the S/H circuit 43A-1.

At the subsequent step S34, after a predetermined time period has passedsince the start of integration (i.e., after passage of the integraltime) while all the light-emitting points A1 to A4 are kept OFF, thecontrol circuit 37 sends the control signal SHS_(dark) to close theswitch on the preceding stage side of the S/H circuit 43A-2 and recordsthe output value of the integrating circuit 42A (after passage of theintegral time) in the capacitor VS_(dark). Thereafter, the controlcircuit 37 sends the control signal SHS_(dark) to open the switch on thepreceding stage side of the S/H circuit 43A-2 and sends the controlsignal RSTa to reset the integrating circuit 42A.

Then, the control circuit 37 sends the control signal SEL to close theswitches on the succeeding stage side of the S/H circuits 43A-1 to43A-4. Accordingly, the subtracting circuit 44-2 subtracts the valueheld by the capacitor VR_(photo) from the value held by the capacitorVS_(photo). Here, the value held by the capacitor VS_(photo) is anoutput value of the integrating circuit 42A which is obtained at thestart of emission of the targeted light-emitting point, and is formed byonly a reset noise component produced at the time of emission. The valueheld by the capacitor VR_(photo) is an output value of the integratingcircuit 42A which is obtained after the integral time has passed sincethe start of emission of the light-emitting point, and includes a resetnoise component at the time of emission and the output value of thephotodetector 41A. Thus, by the above subtraction, an output value ofthe photodetector 41A from which the reset noise has been removed isobtained. This output value is output to the subtracting circuit 44-3 ofthe succeeding stage.

The subtracting circuit 44-1 subtracts the value held by the capacitorVR_(dark) from the value held by the capacitor VS_(dark). Here, thevalue held by the capacitor VS_(dark) represents a reset noise componentin the dark state. The value held by the capacitor VR_(dark) is anoutput value of the integrating circuit 42A which is obtained afterpassage of the integral time while the targeted light-emitting point isin the dark state, and includes a reset noise component in the darkstate and the output value of the photodetector 41A in the dark state(i.e., dark output value). Thus, by the above subtraction, a dark outputvalue from which the reset noise has been removed is obtained. This darkoutput value is output to the subtracting circuit 44-3 of the succeedingstage.

The subtracting circuit 44-3 subtracts the dark output value from theoutput value of the photodetector 41A and outputs the output valueV_(sig) of the photodetector 41A from which the dark output has beenremoved to the ADC 46 of the succeeding stage. The ADC 46 outputs adigital value of the output value V_(sig) of the photodetector 41A fromwhich the dark output has been removed to the control circuit 37. As aresult, the control circuit 37 obtains the photodetection signal A fromwhich the reset noise component and the dark output component have beenremoved. Lastly, the control circuit 37 multiplies the output valueV_(sig) of the photodetection signal A by the temperature correctioncoefficient recorded in the memory section 50 at step S02.

While obtaining the photodetection signal A from which the reset noisecomponent and the dark output component have been removed, the controlcircuit 37 carries out a light quantity correction operation on thephotodetection signal A of the photodetector A4 which has been obtainedin the immediately-previous cycle. In the light quantity correctionoperation, the control circuit 37 calculates the difference between theobtained photodetection signal A and a reference value and calculatessuch a light quantity set value that the difference is zero (0). Thelight quantity set value is overwritten in the memory section 50.

In the foregoing, the process carried out on one of the light-emittingpoints A1 to A4 of the group A has been described with reference to FIG.10. Next, the process carried out on all the light-emitting pointsincluded in the OLED-PH 17 is described. As shown in FIG. 11, thecontrol circuit 37 first sends the control signal SEL to select thegroup A and thereafter selects targets of light quantity detection fromthe light-emitting points A1 to A4 of the group A on a one-by-one basis.Then, as for the selected light-emitting point, the process shown inFIG. 10 is carried out to obtain the output value of the photodetector41A. The control circuit 37 calculates the difference between theobtained output value and an intended value and derives such a lightquantity set value that the difference is zero. The derived lightquantity set value is recorded in a storage section (e.g., register) ofa drive circuit corresponding to a light-emitting point which is thetarget of light quantity detection. Here, correction of the lightquantity set value is performed while the process of FIG. 10 is carriedout for the next light-emitting point. Such an operation is carried outfor each of the light-emitting points A1 to A4.

Then, the control circuit 37 sends the control signal SEL to select thegroup B and thereafter carries out the above process in the same way asdescribed above for the light-emitting points B1 to B4 of the group B.Thereafter, the control circuit 37 sends the control signal SEL toselect the group C and thereafter carries out the above process in thesame way as described above for the light-emitting points C1 to C4 ofthe group C.

Effects

According to the OLED-PH 17 that has the above-described configuration,the temperatures of the photodetectors 41A, 41B, 41C . . . can bedetermined. More specifically, in the OLED-PH 17, the control circuit 37determines the temperatures of the photodetectors 41A, 41B, 41C . . .based on the current value I_(PDA) output from the photodetector 41Awhile the light-emitting points A1 to A4 are OFF. The current valueI_(PDA) output from the photodetector 41A while the light-emittingpoints A1 to A4 are OFF includes a photodiode dark current. Thephotodiode dark current has a temperature dependence. Therefore, thecontrol circuit 37 determines the magnitude of the photodiode darkcurrent from the current value I_(PDA) which is output from thephotodetector 41A while the light-emitting points A1 to A4 are OFF,thereby determining the temperature of the photodetector 41A. As aresult, in the OLED-PH 17, the quantity of light emitted from thelight-emitting points A1 to A4 can be accurately detected inconsideration of the temperature dependence of the photodetector 41A.Note that the same applies to the photodetectors 41B, 41C . . . as tothe photodetector 41A, and therefore, the description thereof is hereinomitted.

According to the OLED-PH 17, the magnitude of the photodiode darkcurrent included in the current value I_(PDA) which is output from thephotodetector 41A while the light-emitting points A1 to A4 are OFF canbe accurately determined. More specifically, the subtracting circuit 44subtracts voltage value V1 held by the capacitor VR_(photo) from voltagevalue V2 held by the capacitor VS_(photo). Here, voltage value V1 is anoutput value of the integrating circuit 42A which is obtained at thestart of integration while the light-emitting points A1 to A4 are OFF,and is formed by the reset noise. Voltage value V2 is an output value ofthe integrating circuit 42A which is obtained after the integral timehas passed since the start of integration while the light-emittingpoints A1 to A4 are OFF, and is formed by the reset noise, theamplification noise, and the photodiode dark current. Therefore, in thesubtracting circuit 44, the reset noise is removed, and the output valueV_(photo) which is formed by the amplification noise and the photodiodedark current can be obtained. Thus, the photodiode dark current can beeasily extracted. Note that the same applies to the photodetectors 41B,41C . . . as to the photodetector 41A, and therefore, the descriptionthereof is herein omitted.

Here, the output value V_(photo) includes the amplification noise. Theamplification noise is random noise. The averaging circuit 48 outputsthe average V_(ave) of eight output values V_(sig) to the controlcircuit 37. By averaging, the amplification noise that is random noiseis removed from the eight output values V_(sig). That is, the controlcircuit 37 can determine the magnitude of the photodiode dark currentand can more accurately determine the temperature of the photodetector41A based on the magnitude of the photodiode dark current. Note that thesame applies to the photodetectors 41B, 41C . . . as to thephotodetector 41A, and therefore, the description thereof is hereinomitted.

When determining the temperature of the photodetector 41A, the OLED-PH17 uses the current values I_(PDA), I_(PDB), I_(PDC) . . . which areoutput from the photodetectors 41A, 41B, 41C . . . while thelight-emitting points A1 to A4, B1 to B4, C1 to C4 . . . are OFF. Sincethe current values I_(PDA), I_(PDB), I_(PDC) which are output from thephotodetectors 41A, 41B, 41C . . . while the light-emitting points A1 toA4, B1 to B4, C1 to C4 . . . are OFF are very small, improving thedetection accuracy of the photodetection signal A requires furtherincreasing voltage value V_(OUT) of the photodetection signal A suchthat a sufficient dynamic range is ensured. In view of such, in theOLED-PH 17, when determining the temperature of the photodetector 41A,the control circuit 37 uses the capacitor CG4 that has the smallestcapacitance.

In the OLED-PH 17, the photodetectors 41A, 41B, 41C . . . are arrangedso as to correspond to respective ones of the light-emitting points A1to A4, B1 to B4, C1 to C4 This arrangement enables the control circuit37 to obtain the temperature distribution of the photodetectors 41A,41B, 41C . . . . Therefore, corrections can be made to thephotodetection signals A, B, C output from the photodetectors 41A, 41B,41C . . . based on the temperatures of the photodetectors 41A, 41B, 41C. . . . As a result, in the OLED-PH 17, the light-emitting points A1 toA4, B1 to B4, C1 to C4 . . . can be accurately controlled to emit lightwith an intended light quantity.

Other Embodiments

The optical writing device according to the present invention is notlimited to the OLED-PH 17 that has been described above but can bevaried within the scope of the spirit of the invention.

Although in the above-described example, in the OLED-PH 17, the totalnumber of the photodetectors 41A, 41B, 41C . . . is smaller than thetotal number of the light-emitting points A1 to A4, B1 to B4, C1 to C4 .. . , the total number of the photodetectors 41A, 41B, 41C . . . may beequal to the total number of the light-emitting points A1 to A4, B1 toB4, C1 to C4 . . . . That is, the photodetectors correspond to thelight-emitting points in a one-to-one manner.

In the OLED-PH 17, the relationship between the integral time in thetemperature correction operation and the integral time in the lightquantity correction operation is not especially mentioned. These may beequal to each other. Alternatively, one may be longer than the other.Note that, however, in the control circuit 37, it is preferred that theintegral time in the case of determining the temperatures of thephotodetectors 41A, 41B, 41C . . . (i.e., in the temperature correctionoperation) is longer than the integral time in the case of determiningthe quantity of light emitted from the light-emitting points A1 to A4,B1 to B4, C1 to C4 . . . (i.e., in the light quantity correctionoperation). In the temperature correction operation, the OLED-PH 17 usesthe current values I_(PDA), I_(PDB), I_(PDC) . . . which are output fromthe photodetectors 41A, 41B, 41C . . . while the light-emitting pointsA1 to A4, B1 to B4, C1 to C4 . . . are OFF. The current values I_(PDA),I_(PDB), I_(PDC) . . . which are output from the photodetectors 41A,41B, 41C . . . while the light-emitting points A1 to A4, B1 to B4, C1 toC4 . . . are OFF are very small. Thus, the OLED-PH can increase thedark-time output signal by increasing the integral time. Note that theOLED-PH 17 may change both the integral time and the gain, or either oneof these, in the temperature correction operation.

Although in the above-described example the averaging circuit 48averages digital output values V_(sig), the averaging circuit 48 mayaverage analog output values V_(sig).

Although in the above-described example the control circuit 37multiplies the output values V_(sig) of the photodetection signals A, B,C . . . by the temperature correction coefficient, the control circuit37 may instead perform an operation using a table, for example.

In the temperature correction operation, the subtracting circuit 44-3subtracts the output value V_(dark) from the output value V_(photo). Theoutput value V_(dark) is zero (0). Therefore, the output value V_(photo)may be employed as the output value V_(sig). In this case, a bypasscircuit is provided in parallel with the subtracting circuit 44-3.

In the temperature correction operation, a plurality of capacitors maybe simultaneously used in the integrating circuit 42A.

The number of capacitors in the integrating circuit 42A is not limitedto four but may be two or more.

The photodetector 41A, the integrating circuit 42A, the S/H circuit 43A,and the subtracting circuit 44 are preferably made of amorphous siliconand/or polysilicon.

Although the present invention has been described in connection with thepreferred embodiment above, it is to be noted that various changes andmodifications are possible to those who are skilled in the art. Suchchanges and modifications are to be understood as being within the scopeof the invention.

What is claimed is:
 1. An optical writing device comprising: a pluralityof light-emitting points; a photodiode configured to output a signalwhich represents a quantity of incident light from a predeterminedlight-emitting point selected from the plurality of light-emittingpoints; and a calculation section for calculating a temperature of thephotodiode based on a magnitude of a photodiode dark current included inthe signal output from the photodiode while the predeterminedlight-emitting point is OFF, the calculation section includes acontroller configured to determine the magnitude of the photodiode darkcurrent based on an output value of an integrating circuit forintegrating an output of the photodiode, the integrating circuitincluding a resettable capacitor, and wherein the integrating circuitincludes a plurality of capacitors which have different capacitances,and the controller uses one of the capacitors which has the smallestcapacitance for determination of the temperature of the photodiode. 2.The optical writing device according to claim 1, wherein the pluralityof light-emitting points are arranged in a predetermined direction. 3.The optical writing device according to claim 1, wherein each of thelight-emitting points is an OLED.
 4. The optical writing deviceaccording to claim 1, wherein the predetermined light-emitting pointemits light to scan a photoreceptor.
 5. The optical writing deviceaccording to claim 4, wherein plural ones of the photodiode are providedso as to correspond to respective ones of the light-emitting points. 6.The optical writing device according to claim 4, wherein a total numberof the photodiodes is smaller than a total number of the light-emittingpoints.
 7. An image forming apparatus comprising the optical writingdevice as set forth in claim
 1. 8. An optical writing device comprising:a plurality of light-emitting points; a photodiode configured to outputa signal which represents a quantity of incident light from apredetermined light-emitting point selected from the plurality oflight-emitting points; and a calculation section for calculating atemperature of the photodiode based on a magnitude of a photodiode darkcurrent included in the signal output from the photodiode while thepredetermined light-emitting point is OFF, wherein the calculationsection includes an integrating circuit for integrating an output of thephotodiode, the integrating circuit including a resettable capacitor, aS/H circuit capable of recording a first output value obtained at astart of integration in the integrating circuit while the predeterminedlight-emitting point is OFF and a second output value obtained after apredetermined time period has passed since the start of integration inthe integrating circuit, a subtracting circuit for subtracting the firstoutput value from the second output value to output a third outputvalue, and a controller configured to determine the magnitude of thephotodiode dark current based on the third output value.
 9. The opticalwriting device according to claim 8, wherein the calculation sectionfurther includes an averaging circuit configured to output an average ofplural ones of the third output value output from the subtractingcircuit, and the controller accepts the average of plural ones of thethird output value as the magnitude of the photodiode dark current. 10.The optical writing device according to claim 8, wherein the calculationsection further includes a memory section for storing a table whichrepresents a relationship between the magnitude of the photodiode darkcurrent and the temperature of the photodiode, and the controller refersto the table to determine the temperature of the photodiode.
 11. Theoptical writing device according to claim 8, wherein the controllerdetermines the temperature of the photodiode by an interpolationoperation.
 12. The optical writing device according to claim 8, whereinthe integrating circuit includes a plurality of capacitors which havedifferent capacitances, and the controller uses one of the capacitorswhich has the smallest capacitance for determination of the temperatureof the photodiode.
 13. The optical writing device according to claim 8,wherein the S/H circuit is capable of recording a fourth output valueobtained at the start of integration in the integrating circuit whilethe predetermined light-emitting point is ON and a fifth output valueobtained after a predetermined time period has passed since the start ofintegration in the integrating circuit, and the controller determines aquantity of light emitted from the predetermined light-emitting pointbased on the first output value, the second output value, the fourthoutput value, and the fifth output value.
 14. The optical writing deviceaccording to claim 13, wherein the controller sets the predeterminedtime period for determination of the temperature of the photodiode so asto be longer than the predetermined time period for determination of thequantity of light emitted from the predetermined light-emitting point.15. A temperature calculation method for determining, in an opticalwriting device including a plurality of light-emitting points, atemperature of a photodiode configured to output a signal whichrepresents a quantity of incident light from a predeterminedlight-emitting point selected from the plurality of light-emittingpoints, the method comprising the steps of: acquiring a signal from thephotodiode while the predetermined light-emitting point is OFF; anddetermining the temperature of the photodiode based on a magnitude of aphotodiode dark current included in the signal, wherein the magnitude ofthe photodiode dark current is based on an output value of anintegrating circuit for integrating an output of the photodiode, theintegrating circuit including a resettable capacitor, and theintegrating circuit includes a plurality of capacitors which havedifferent capacitances, and using one of the capacitors which has thesmallest capacitance for determination of the temperature of thephotodiode.
 16. A temperature calculation method for determining, in anoptical writing device including a plurality of light-emitting points, atemperature of a photodiode configured to output a signal whichrepresents a quantity of incident light from a predeterminedlight-emitting point selected from the plurality of light-emittingpoints, the method comprising the steps of: acquiring a signal from thephotodiode while the predetermined light-emitting point is OFF; anddetermining the temperature of the photodiode based on a magnitude of aphotodiode dark current included in the signal, wherein thedetermination of the magnitude of the photodiode current in the signalcomprises: integrating an output of the photodiode with an integratingcircuit, the integrating circuit including a resettable capacitor,recording with a S/H circuit, a first output value obtained at a startof integration in the integrating circuit while the predeterminedlight-emitting point is OFF and a second output value obtained after apredetermined time period has passed since the start of integration inthe integrating circuit, subtracting the first output value from thesecond output value to output a third output value, and determining themagnitude of the photodiode dark current based on the third outputvalue.
 17. The temperature calculation method according to claim 16,comprising: arranging the plurality of light-emitting points in apredetermined direction.
 18. The temperature calculation methodaccording to claim 16, wherein the predetermined light-emitting pointemits light to scan a photoreceptor.
 19. The temperature calculationmethod according to claim 18, wherein plural ones of the photodiode areprovided so as to correspond to respective ones of the light-emittingpoints.
 20. The temperature calculation method according to claim 18,wherein a total number of the photodiodes is smaller than a total numberof the light-emitting points.